The invention relates to a feed bushing for single-screw extruders which consists of a grooved bushing, an outer casing and a cooling system.
The processing of very high-molecular-weight and high-viscosity thermoplastics in single-screw extruders today is usually done only in extruders with a positive-conveying feed section. This system offers a great many advantages over conventional extrusion. Among these are high output at low melting temperatures and high melt pressures; reduced shear of the plastics, which makes this system energy-saving and very economical; gentle plastication of the plastics; no thermal or chemical degradation, hence optimum product quality; and particularly stable extrusion so that output, pressure and melting-temperature surges are prevented.
With this system, a very high efficiency of polymer movement and melt-pressure buildup is achieved by the use of a water-cooled feed section with a plurality of conically expanding grooves which are uniformly distributed in the bore of the feed section over a length of about three diameters.
In this system, so-called mixing-section screws having shearing and mixing sections are used as best suited. The ejection cylinder comprising one or more heating and cooling collars is flanged directly to the feed section.
However, with this extruder design comprising a water-cooled and grooved feed bushing, satisfactory and trouble-free operation is assured only when the plasticating elements in the feed section are extremely wear-resistant. Now in the extrusion of high-molecular-weight and colored polymers, the wear is caused by the hard pigments and the high pressures in the grooved zone. The abrasive wear of the surfaces increases with the melt pressure in the grooved zone (which may be as high as 2000 kg/cm.sup.2) and with the hardness of the pigments (Vickers hardness numbers of up to 1800).
A further requirement is that the feed section be vigorously water-cooled for removal of the heat of friction of the granules in the grooved zone. However, depending on the hardness of the cooling water and the degree to which it carries impurities, the cooling channels may become encrusted with calcium deposits even after a short time.
Experience has shown that when the plasticating elements close up or the cooling channels become clogged the extruder cannot be relied on to work properly. Serious malfunctions will then occur in short order. The conveying efficiency then falls off sharply, with the result that the output decreases, the melting temperature rises and the melt pressure drops. The plastic then undergoes thermal and chemical degradation. Moreover, extrusion then is no longer steady but pulsating. Production with a worn-out or calcium encrusted feed bushing thus is not possible, and the extruder is not fit for use.
One way of solving the wear problem is to use deep-hardened screws and grooved bushings made of tool steel. In the case of extruders which are subjected to very high loads, and in the handling of highly abrasive plastics, the grooved inset may even be fabricated from cemented carbides.
Feed bushings consisting of a grooved bushing made of a wear-resistant material and having conically expanding grooves in their bore are known.
The feed bushing with the cooling channel on the outer periphery thereof is shrink-fitted into the outer casing. Through bores in the latter serve for mounting the feed bushing on the extruder drive.
With this design, to prevent fracture of the grooved bushing the allowance for shrinkage to the outer casing must be very high so that with maximum melt pressures in the screw of up to 2000 kg/cm.sup.2 the grooved bushing is always prestressed for pressure.
In the fabrication of a grooved bushing from hardened steel or cemented carbide, the shrinkage allowance is set so high that the grooved bushing cannot later be forced out of the outer casing.
Calcium-encrusted or clogged cooling channels then cannot be cleaned out without complete destruction of the outer casing. Depending on the hardness of the cooling water and the degree to which it carries impurities, such a feed bushing may become inoperative after a very short operating time because of inadequate water cooling and must then be replaced with a complete new feed bushing even though no wear is discernible in its bore.